The effect of the parasite Toxoplasma gondii on the sex ratio of human beings

As mentioned in Chapter 2 in the book, many people are infected by the parasite Toxoplasma gondii throughout their lives. In developed countries, 15–40% of women of reproductive age are usually infected; in developing countries with lower hygienic standards, the occurrence of this “latent” toxoplasmosis approaches 90%. A study that we recently performed on a large number of women indicated that far more boys than girls are born to infected women in their first pregnancy. In a set of 111 women with the highest antibody levels (i.e. with the strongest or freshest, nonetheless latent, infection), the ratio of boys to girls attained a value of 2.6:1. We observed a similar phenomenon in experiments performed on infected mice. It is not yet known in which way toxoplasma affects the ratio of the sexes of human beings. However, it seems most probable that the protozoa in some way reduces the probability that the embryos of individuals of male sex will be aborted in the first weeks of pregnancy. It is well known that male embryos have a much better chance of implantation in the uterus of the mother than female embryos, but that they simultaneously have a much greater chance that they will be aborted in the first weeks of pregnancy. Of a ratio of the sexes of 1.64:1 in favour of boys in the 5–7th week of pregnancy, the secondary sex index usually decreases by the time of parturition to the usual value of 1.06:1, corresponding to 106 newborn boys to 100 newborn girls. The immune system of the mother plays an important role in elimination of male embryos as it recognizes antigens specific for male cells, H–Y antigens. It is known that toxoplasma has a substantial impact on immune processes occurring in the infected organism. Thus, it is possible that toxoplasma can affect the ratio of the sexes in favour of boys by suppressing the component of the immune system that is responsible for elimination of male embryos. In conclusion, two questions to make you think: Older parasitologists observed that, in a population in which about 30% of the individuals are infected by toxoplasma, more than 80% of children with Down’s syndrome are born to mothers infected by latent toxoplasmosis. Modern parasitologists and physicians, of course, laughed at this – we obviously know that Down’s syndrome is not caused by a parasitic protozoa but by the fact that two copies of chromosome number 21 accidentally entered the egg during meiosis and the individual was created by the fertilization of this egg by normal sperm so that the individual has three copies of this chromosome in their cells instead of two. The first question – how could toxoplasma lead to increased frequency of children with Down’s syndrome in infected mothers, without having to attack the future sex cells and play around with their chromosomes during meiosis? Second question, far more difficult, to which I also do not know the answer – should parents who are taking care of a beloved child with Down’s syndrome curse toxoplasma or thank it?

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more